Advanced energy storage systems such as lithium-ion batteries are important approaches to mitigate energy shortage and global climate warming issues that the world is currently facing. High power and high energy density are essential to batteries for applications in electric vehicles, stationary energy storage systems for solar and wind energy, as well as smart grids. Because conventional lithium-ion batteries are inadequate to meet these needs, advanced materials with high capacity and fast charge-discharge capability are desirable for next generation lithium-ion batteries. Titanium dioxide (TiO2) and various polymorphs (anatase, rutile, and TiO2—B (bronze)) have been widely investigated as lithium-ion battery anode materials, due to their advantages in terms of cost, safety, and rate capability. In particular, the polymorph of TiO2—B has shown a favorable channel structure for lithium mobility, which results in fast charge-discharge capability of a lithium cell. It has been identified that the lithium intercalation in TiO2—B features a pseudocapacitive process, rather than the solid-state diffusion process observed for anatase and rutile. Theoretical studies have uncovered that this pseudocapacitive behavior originates from the unique sites and energetics of lithium absorption and diffusion in TiO2—B structure. As a result, TiO2—B nanoparticles, nanotubes, nanowires, and nanoribbons have been reported as anode materials with good rate performance for lithium-ion batteries. These nanomaterials displayed attractive battery performance; however, they also have some disadvantages, e.g., poor electronic conduction network due to aggregation of nanopowders, loss of particle connection during cycling, and low packing density.
Mesoporous materials (LiFePO4 and TiO2) with micrometer-sized particles were found to be able to overcome these shortcomings, yet still maintain the advantages of nanomaterials. The properties of mesoporous materials ensure high contact area between electrolyte and electrode, short diffusion distances for Li+ transport, and good accommodation of strain during cycling. The general concern for mesoporous materials is the long transport distance of electrons in micrometer sized particles. Conductive carbon and RuO2 coatings have thus been employed to improve the high rate performance of lithium storage in mesoporous TiO2 materials.
However, one concern for mesoporous materials is the long transport distance of electrons in micron sized particles, especially for low conductivity TiO2. Therefore, improving the electronic conductivity of TiO2 may be important for such mesoporous metal oxide (e.g., TiO2) materials. One method of increasing the electronic conductivity of TiO2 is to modify the bandgap of a pure TiO2 by different doping schemes, such as iron-, tungsten-, or nitrogen-doped TiO2 nanoparticles and nanotubes, have been used as anode materials for lithium-ion batteries and confirmed to exhibit better capacity and rate capability. However, an inherent obstacle relates to the fact that the thermodynamic solubility for substitutional doping of TiO2 is extremely low for most dopants. Doping would be of no help in providing mobile charge carriers if the dopants are mainly located at undesirable interstitial sites as opposed to substitutional doping. Recently a non-compensated n-p co-doping concept has been reported to enable to enhance the visible-light photoactivity of TiO2 by narrowing its band gap. Therefore, it is highly desirable to develop a treated (e.g., doped) metal oxide compound with the morphology of mesoporous microspheres that could combine the advantages of mesoporous structure, spherical morphology, and improved electronic conductivity.